1. Field of the Invention
The present invention relates to a multi-layered holographic read-only memory, preferably used as a mobile (or portable) memory card such as a magnetic card and an IC card. This type of card is difficult to forge or reproduce, and thus can be used as an authentication card for electronic commercial transaction. Additionally, the present memory has a large capacity and can be manufactured at low cost, and thus is suitable for distributing software for music, pictures, computer applications, or the like.
This application is based on patent applications Ser. Nos. Hei 10-32578, Hei 10-44941, and Hei 10-75336 filed in Japan.
2. Description of the Related Art
Magnetic cards such as a telephone card are conventionally used as mobile (or portable) information cards which a user can carry in a pocket. Recently, IC cards have become practical to use, and application of the IC cards to the electronic commercial transaction has been investigated. The magnetic cards are cheap, but may be forged. The IC cards are difficult to forge, but their cost per bit is expensive.
The holographic storage technique is another technique applicable to a data memory which is difficult to forge and has a large capacity. Holography can be classified into thin film holography and volume holography.
A volume holographic memory has a larger storage capacity; however, no data duplication technique suitable for the volume hologram exists. Therefore, this type does not suit mass production, and it is difficult to apply the volume holography to a read-only memory used as an authentication card or used for distributing software applications.
A thin film hologram can be mass-produced using a printing technique, but has a limited memory density. Therefore, in consideration of the size and convenience of a necessary data retrieval device, the memory using thin film holography has less appeal in comparison with the IC card. Even if such thin film holograms are multi-layered so as to solve the above problem and to increase the storage capacity, holographic images, reconstructed from each hologram by using an ordinary reconstruction method, are simultaneously reconstructed. Therefore, due to the crosstalk being large, necessary data or information cannot be obtained.
An objective of the present invention is to provide an inexpensive multi-layered holographic read-only memory having a large capacity, which is applicable to a mobile card or a storage medium used in a data storage device. Another objective is to provide a multi-layered holographic read-only memory by which the data-retrieval speed can be improved.
In order to realize the above objective, the present invention provides a multi-layered holographic read-only memory in which single-mode slab waveguides are stacked to be multi-layered, wherein a periodic scattering center whose period approximately agrees with the period of the guided mode is provided in at least one of a core layer and a clad layer in each waveguide so that a guided wave in the waveguide is diffracted by the periodic scattering center to the outside of the waveguide and a holographic image is generated.
That is, the principle of the thin film holography is used, and the hologram based on this is difficult to forge while it can be mass-produced. In the present invention, such thin film holograms are multi-layered, and the holographic image of each layer can be individually reconstructed.
In the general thin film holography, even if the incident direction or wavelength of a beam for reconstructing the image (i.e., a reference beam) is changed so as to change the position, magnification, or diffraction direction of the reconstructed image, the reference beam is always diffracted. That is, in the multi-layered thin film holograms, as far as the reference beam reaches each thin film hologram, crosstalk is inevitable regardless of the incident direction of the reference beam. In the present invention, each thin film hologram is embedded in the waveguide and the guided wave functions as a reference beam, thereby preventing the reference beam from reaching holograms other than the target hologram.
Therefore, in the present invention, the data storage capacity can be increased like the volume holographic memories while the principle of the thin film holography suitable for mass production can be used. Accordingly, an inexpensive read-only memory having a larger storage capacity can be realized. If the present memory is applied to a portable memory card, a rotating mechanism employed in the optical disc or the like is unnecessary; thus, power necessary for the retrieval device (for retrieving musical or video data) can be reduced. If the present memory is applied to an authentication card, the card is difficult to forge and various additional information data can also be stored therein. Therefore, convenience can be improved.
Typically, at least one of the edges of the multi-layered slab waveguide is cut so as to produce a reflecting surface which is slanted (or inclined) by approximately 45xc2x0 with respect to the normal direction of the waveguide plane, and light is incident from the direction substantially perpendicular to the waveguide plane on the reflecting surface so as to introduce the light into the waveguide. If the multi-layered slab waveguide has opposite edges, both the edges may function as 45xc2x0-cut reflecting surfaces, and light-introducing (or coupling) positions can be determined such that guided waves incident from these edges do not overlap with each other in the relevant waveguide plane. Furthermore, a plurality of the multi-layered slab waveguides may be placed and bonded with each other in the waveguide plane so as to make a card.
According to such variations, possible areas for storing readable data as holograms can be increased; thus, the storage capacity can be increased, and the data retrieval speed can be improved. If the present invention is applied to a portable card, the effective storage area can be increased depending on the total area of the card.
On the other hand, the multi-layered slab waveguide may have a disc shape, and a light-introducing section may be provided in a central area of the disc, so as to guide light towards the outer circumference of the disc.
Typically, the light-introducing section is a reflecting surface having the shape of a 45xc2x0-slanted side face of a cone, and light can be incident from the direction substantially perpendicular to the disc plane on the reflecting surface so as to introduce the light into the waveguide.
In this case, the following arrangement is possible: a plurality of coupling points for introducing light are concentrically and periodically placed in the reflecting surface; a guided wave is transmitted from each coupling point towards the outer circumference of the disc while the guided wave expands as a fan-shape having a predetermined angle of expansion; and the predetermined angle of expansion is determined in order that fan-shaped portions corresponding to each coupling point do not overlap with each other.
In the above arrangement, the coupling points are circularly concentrated near the center of the disc. Therefore, with a single head above the circumference of the circle of the coupling points, the waveguide of each fan-shape portion can be accessed in turn by rotating the disc, and it is efficient.
Generally, a rotational data-retrieval device has a much simpler structure and a higher access speed in comparison with a linear mechanical-motion data-retrieval system. When the target layer is changed, the lens of a light source must be moved also in this case (like the card type). However, in the rotational data-retrieval device, the requirement for the necessary moving distance is 1 mm at the most. A precise micro-motion mechanism (i.e., actuator) necessary for such a stroke is widely used for optical discs or the like, and is not expensive. In addition, such a micro-motion mechanism has a short response time of approximately 1 ms.
When data stored in the disc memory are retrieved by rotating the disc, it is possible to extract a non-diffracted portion of the guided wave outside of the memory so as to establish a synchronous condition in the rotation.
Consequently, according to the present invention, both the possible data-storage area of the hologram and the data-retrieval speed can be remarkably improved.
Japanese Patent Application, First Publication, No. Hei 9-101735 discloses a data retrieval technique using multi-recorded holograms. In this case, a part of or the whole of the holograms is made of an optical recording material, and data are recorded using the optical interference. Data are retrieved using a reference beam. However, every object beam passes through all the layers during the recording of each hologram. Here, exposure of a specific intensity must be performed so as to obtain the necessary signal intensity. In this method, the S/N ratio is decreased in inverse proportion to the square of the number of layers. In contrast, in the present invention, the hologram is provided in advance for each target layer, and data are retrieved using the guided wave, as described above; thus, the S/N ratio is merely decreased in inverse proportion to the number of layers. That is, Hei 9-101735 has different structure and functions in comparison with the present invention, and has an essential problem with respect to the multi-layered structure.